A superfluid gyroscope with cold atomic gases

11 Pages·0.11 MB·English

by  S. Stringari

A SUPERFLUID GYROSCOPE WITH COLD ATOMIC GASES
S. Stringari
Dipartimento di Fisica, Universita` di Trento,
and Istituto Nazionale per la Fisica della Materia,
I-38050 Povo, Italy
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0 (February 1, 2008)
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J Abstract

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1
v AtrappedBose-Einstein condensedatomic gas containingaquantizedvortex
3
1 is predicted to exhibit precession after a sudden rotation of the confining
1
1
potential. The equations describing the motion of the condensate are derived
0
1
0 and the effects of superfluidity explicitly pointed out. The dependence of the
/
t
a
precession frequency on the relevant parameters of the problem is discussed.
m
- Theproposed gyroscope is well suited to explore rotational effects at the level
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o of single quanta of circulation.
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: PACS numbers: 03.75.Fi, 05.30.Jp, 32.80.Pj, 67.40.-w
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Typeset using REVTEX
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After the realization of Bose-Einstein condensation [1] in magnetically trapped vapours
of alkali atoms, cooled down to extremely low temperatures, the superfluid behaviour of
these systems has become the object of extensive experimental work. This includes the
study of rotational properties, like quantized vortices [2,3] and the quenching of the moment
of inertia [4], as well as the reduction of dissipative effects [5].
The purpose of this work is to show that Bose-Einstein condensed (BEC) gases can be
used to realize a quantum gyroscope where the effects of superfluidity show up in a very
peculiar way. Superfluid gyroscopes have been already realized with liquid 4He [6] and 3He
[7] and have been mainly used to investigate the nature of persistent currents in toroidal
geometries. Most of experiments with helium gyroscopes operate with many quanta of
circulation. Compared to liquid helium, trapped BEC gases are mesoscopic systems in the
sense that the healing length, which provides a typical range of dynamic correlations and
fixes the size of the vortex core, is smaller, but not extremely smaller than the size of the
sample [8]. For the same reason the angular momentum Nh¯ carried by a single quantized
vortex can have visible effects on the global motion of the condensate as we will prove in
this letter. A further feature that characterizes these systems is the very peculiar type of
confinement which yields new possibilities for exploring superfluid phenomena. Important
gyroscopic effects associated with the occurrence of vortex lines have been already observed
intrappedBECgases[9–11]. Theauthorsof[9]havesucceeded intestingthequantizationof
the angular momentum of a single vortex line. To this purpose one generates a quadrupole
deformation in the plane orthogonal to the vortex axis. The observed precession of the
deformation is proportional to the angular momentum carried by the vortex, in accordance
with the predictions of theory [12,13]. Notice that in this experiment the vortex line is not
affected by the precession. The authors of [10] and [11] have instead observed the precession
of a vortex line either displaced or tilted from the symmetry axis of the condensate. In this
case the motion of the condensate is not affected at a macroscopic level and the precession
involves the change of the density on a more microscopic scale, of the order of the size of
the vortex core.
2
In the present work we discuss the macroscopic precession of the symmetry axis of a
deformedcondensatecausedbythesuddenrotationofthetrap,inthepresenceofaquantized
vortex line. This precession corresponds to a full rotation of the condensate in 3D, which
preserves the intrinsic shape of the system.
The geometry of the proposed gyroscope is illustrated in fig.1. The system consists of a
dilute and cold gas of atoms with mass m confined by an axi-symmetric trap of harmonic
shape:
m
2 2 2 2 2
V = ω (x +y )+ω z (1)
ext 2 ⊥ z
(cid:16) (cid:17)
where ω⊥ and ωz are, respectively, the radial and axial trapping frequencies. The trapping
potential is characterized by the deformation parameter
(ω2 ω2)
ǫ = ⊥ − z (2)
(ω2 +ω2)
⊥ z
which will be taken different from zero. As a consequence of this anisotropy the atomic
cloud is also deformed. If the atoms are bosons, at very low temperatures they undergo
a phase transition associated with the macroscopic occupation of a single-particle state
(Bose-Einstein condensation). This happens for temperatures below the critical value kT =
c
0.94h¯ω N1/3 where ω = (ω ω2)1/3 is the geometrical average of the trapping frequencies
ho ho z ⊥
and N is the number of atoms. Typical values of T range between 0.1 and 1µK. Below T
c c
the system exhibits unique features which have been the object of systematic experimental
and theoretical investigation. For our purposes the system should be very cold in order to
minimize the dissipative effects produced by the thermal cloud which tend to destabilize
the vortex line [15,16]. For this reason we will discuss here the behaviour of the gas at zero
temperature.
Let us consider a Bose-Einstein condensate in equilibrium in the trap, with no angular
momentum, and let us suppose that the symmetry axis of the trapping potential is suddenly
rotated in the xz plane through an angle θ . After the rotation the gas is no longer in
0
equilibrium and starts oscillating around the new symmetry axis. If the angle of rotation
3
is small the gas will keep its intrinsic shape and the motion will correspond to a periodic
oscillation characterized by the angle θ(t), giving rise to the so-called scissors mode [14].
The motion is very different from the one of a classical body because the inertia is strongly
suppressed by superfluidity. In particular the oscillation frequency does not vanish when
the deformation (2) of the trap becomes small, as one would expect for a classical system.
The calculation of the frequency of the scissors mode requires the solution of a non trivial
many-body problem where the interactions among particles and the effects of quantum
statistics play an important role. An exact solution of the problem can be obtained for
large samples, such that the dimensionless parameter Na/a is much larger than unity.
ho
This limit corresponds to the so-called Thomas-Fermi (TF) regime and is well achieved
in many available experimental configurations. Here a is the s-wave scattering length (of
the order of 10−2 10−3µm) while a = h¯/(mω ) is the oscillator length fixed by the
ho ho
−
q
geometrical average of the trapping frequencies (of the order of a few microns in standard
magnetic traps). In the TF limit, the equilibrium density takes the form of an inverted
parabola: n(r) = m[µ V (r)]/(4πh¯2a) for µ V and zero elsewhere, where µ =
ext ext
− ≥
(ω /2)(15Na/a )2/5 is the chemical potential fixed by the normalization of the density.
ho ho
The surface where the density vanishes defines an ellipsoid whose principal radii R⊥ and
R are determined by the relationship ω2R2 = ω2R2 = 2µ/m. In the Thomas-Fermi limit
z ⊥ ⊥ z z
the equations of motion take the simplified form of the hydrodynamic theory of superfluids
[17]. With respect to the equations of classical hydrodynamics they are characterized but
the additional constraint of irrotationality. It is worth noticing that the ”hydrodynamic”
form of these equations is not the result of collisional processes as happens in classical gases,
but is the consequence of superfluidity. These equations admit an analytic solution for the
scissors mode with frequency [14]
ω = ω2 +ω2 (3)
scissors ⊥ z
q
This mode has recently been observed [4] confirming the predictions of theory with high
accuracy.
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Let us discuss now the case in which the condensate has initially an intrinsic angular
momentum. This can be due to a quantized vortex aligned along the symmetry axis of the
condensate, carrying one unit h¯ of angular momentum per particle. The size of the vortex
core is fixed by the healing length ξ = (2mµ/h¯2)−1/2 and is always smaller than the size
of the condensate. Also in the presence of the vortex the sudden rotation of the trap will
excite the scissors mode, but now the oscillation will precess due to the torque
d 1
< L >= < [L,H] >= m < r V > (4)
ext
dt ih¯ − ×∇
produced by the external anisotropic potential (1). Here L is the angular momentum op-
erator and the expectation value is taken on the quantum mechanical state of the system.
The value of the torque along the y axis is d < L > /dt = m(ω2 ω2)N < xz > and
y − ⊥ − z
differs from zero immediately after the sudden rotation of the trap in the xz-plane, thereby
causing the precession.
In order to derive the equation for the rotation of the condensate we notice that, if the
angle of rotation θ is small compared to the deformation (2) of the trap, the shape of the
0
sample is preserved. Under this condition the whole motion can be described in terms of
two angles: the inclination angle θ and the azimuthal angle φ. This is a remarkable feature
because gases are highly compressible and their shape can change very easily. The values of
θ and φ are related to the averages < xz > and < yz > by the geometrical relations
R2ω2 ω2 R2ω2 ω2
< xz >= z − ⊥θcosφ , < yz >= z − ⊥θsinφ (5)
7 ωzω⊥ 7 ωzω⊥
where R = √R⊥Rz and we have assumed θ ǫ.
≪
The crucial problem now is to obtain the equations for the averages < xz > and < yz >
inthepresence ofthevortex. This problemwas solved in[12]where it wasshown that, inthe
Thomas-Fermi limit, the quadrupole operators q± = Nk=1(xk ±iyk)zk excite, respectively,
P
just one collective mode > with frequency ω±. The solution of the Schr¨odinger equation,
| ±
in the presence of a small perturbation generated by these operators, then takes the form
Ψ(t) >= e−iE0t/¯h 0 > +c+e−iω+t + > +c−e−iω−t > (6)
| | | | −
h i
5
where 0 > is the equilibrium configuration, > are eigenstates of the Hamiltonian
| | ±
corresponding to the elementary excitations created by q± and the complex parameters c±
characterize the initial state of the system. One can also show that the strengths of the
quadrupole operators q± are equal: < 0 q−q+ 0 >=< 0 q+q− 0 >, while the splitting
| | | |
between the two frequencies takes the simple form [12]
< l >
z
ω+ −ω− = m < x2 +z2 > (7)
In eq.(7) the quantity < ℓ > is the angular momentum per particle in the equilibrium
z
configuration, while the average square radii can be easily evaluated in the Thomas-Fermi
limit where one finds < x2 >=< y2 >= (2/7)µ/mω2 and < z2 >= (2/7)µ/mω2. Result
⊥ z
(7) holds if the splitting is small compared to the unperturbed value (3) of the collective
frequency.
The scenario emerging from the above discussion is quite clear. In the absence of vortices
(< l >= 0) the two modes > are degenerate and their frequency is given by (3). This is
z
| ±
a simple consequence of time reversal symmetry. In the presence of the vortex there is a lift
of degeneracy which is determined by the angular momentum of the system according to (7)
[18]. By expressing the average values < xz > and < yz > in terms of the expectation values
of the operators q± and using explicitly eq. (6), one derives the following time dependence
for the angles θ and φ:
′ ′
θ(t)cosφ(t) = αcosω+t+α sinω+t+βcosω−t+β sinω−t (8)
′ ′
θ(t)sinφ(t) = α cosω+t+αsinω+t+β cosω−t βsinω−t (9)
− −
′ ′
where the real coefficients α, α, β and β depend on the initial conditions for θ and φ. For
′
example the normal modes > can be separately excited by choosing α = θ , α = β =
0
| ±
′ ′ ′
β = 0 and β = θ , α = α = β = 0 respectively. In both cases the inclination angle θ
0
remains constant, θ(t) = θ , while the azimuthal angle φ precesses according to the laws ω t
0 +
and ω−t, respectively.
−
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In the most relevant case of a sudden rotation in the xz plane, the initial conditions
′ ′
instead correspond to θ(0) = θ ,θ (0) = φ(0) = φ(0) = 0. In this case the parameters of
0
′ ′
eqs.(8-9) are α = β = θ /2, α = β = 0 and the solutions take the form
0
(ω+ ω−)t
φ(t) = − (10)
2
and
(ω+ +ω−)t
θ(t) = θ cos (11)
0
" 2 #
The simultaneous excitation of the > modes is easily understood by noticing that
| ±
the rotation of the trap produces a change in the external potential of the form δV =
ext
m(ω2 ω2)θ xz. This gives rise to a perturbative term in the Hamiltonian, proportional to
⊥− z 0
thecombination(q++q−) ofthequadrupoleoperatorsintroduced above. Equations(10)and
(11)showthatthescissorsoscillation, characterized bythefrequency (ω++ω−)/2 ωscissors,
∼
undergoes a precession dφ/dt fixed by the splitting (7). The relative precession can be
expressed in the form
ω+ ω− 7< ℓz > λ5/3 a −2/5
− = 15N (12)
ω+ +ω− 2 h¯ (1+λ2)3/2 (cid:18) aho(cid:19)
where λ = ωz/ω⊥ = (1 ǫ)/(1+ǫ) and we have used the Thomas-Fermi result for < x2 >
−
q
and < z2 >. The precession frequency dφ/dt, and hence the ratio (12), depends explictly
on the value of the angular momentum per particle which, in the case of a single quantized
vortex aligned along the symmetry axis of the condensate, is given by < ℓ >= h¯. The ratio
z
(12) depends also on the shape of the trap. This provides further flexibility to optimize
the visibility of the precession. In typical experimental configurations the Thomas-Fermi
combination (15Na/a )−2/5 is of the order of 10−2 so that for a highly elongated trap
ho
(λ 1) the relative precession is small. For values of λ closer to unity the ratio (12)
≪
becomes larger. With suitable choices of the parameters of the trap the precession can be
easily of the order of 1 Hz and should be consequently observable by imaging the atomic
cloud at different times.
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In conclusion we have shown that Bose-Einstein condensed gases confined in harmonic
traps can be used to realize a quantum gyroscope characterized by two important superfluid
effects: the reduced value of the inertia of the sample and the quantization of the angular
momentum associated with the vortex. The proposed gyroscope is characterized by the
precession of the symmetry axis of the condensate around the symmetry axis of the con-
fining trap, the precession frequency being fixed by the angular momentum carried by the
vortex line. The experimental realization of the proposed gyroscope would provide a further
tool to explore the intriguing features exhibited by rotating Bose-Einstein condensed gases,
including the stability [19,20] and the life time [16] of vortex lines.
It is a pleasure to thank E. Cornell, J. Dalibard, A. Fetter, R. Packard, J. Reppy, E.
Varauqaux and J. Vinen for useful comments and discussions. I also like to thank the
hospitality of the Lorentz Center in Leiden and of the Newton Institute in Cambridge. This
research is supported by the Ministero della Ricerca Scientifica e Tecnologica (MURST).
8
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[18] A similar scenario takes place if we excite the system with the quadrupole operators
N (x iy )2. This choice corresponds to generating a quadrupole deformation in the
k=1 k± k
P
plane perpendicular the symmetry axis, like in the experiment of [9]. In this case the
unperturbed frequency of the quadrupole oscillation is √2ω⊥, while the splitting takes
the form ω+ ω− = (2/m) < ℓz > /(< x2 > + < y2 >) [12].
−
[19] A. Svidzinsky and A. Fetter, Phys. Rev. Lett. 84 5919 (2000) and e-print cond-
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[20] J.J. Garcia-Ripoll and V.M. Perez-Garcia, cond-mat/0012071.
FIGURE CAPTION:
Schematic picture of the BEC gyroscope. The condensate is suddenly rotated with respect
to the symmetry axis of the trap (z-th axis) and exhibits precession in the presence of a
quantized vortex line.
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